In a method for efficiently amplifying a signal having an amplitude-modulated component without distortion, an envelope feedback is employed.
FIG. 1 shows an example of prior art linear amplifier circuit employing the envelope feedback. This prior art is shown, for example, in Electronics Letters, Apr. 8th, 1971, Vol. 7, No. 7, pp. 145 to 146. Referring to FIG. 1, a situation in which a QAM (quadrature amplitude modulated) signal is amplified. The QAM signal contains information of both the amplitude component and the phase component. It is therefore necessary to perform the amplification with a high fidelity with respect to the amplitude and the phase components in order to eliminate distortion in amplification.
A QAM signal input to the input terminal 10 is divided into two parts at a powder divider 1 (or a hybrid). One part is sent to a limiter 2, and the other part is input to an input signal detector 5. In the limiter 2, the amplitude component of the signal is removed, and its output is input to a saturation-type amplifier (non-linear amplifier) 3, where the signal is power-amplified and the amplified signal is output via an output terminal 11. The output power of the saturation-type amplifier 3 is controlled in accordance with the voltage applied to a power control terminal 9. Part of the output power of the saturation-type amplifier 3 is derived by means of a coupling capacitor, a coupling stripe line or the like, and input to and detected by an output signal detector 4. The output of the output signal detector 4 is an envelope component of the output power, i.e., a signal proportional to the amplitude component, which is passed through a DC amplifier (.beta.1) 6 to a first input terminal of the comparator 8. The comparator 8 determines the difference between the signals input to the first and the second input terminals and applies the difference as a feedback voltage, after adding it to the input signal on the first input terminal, to the power control terminal 9 of the saturation-type amplifier 3.
That is, the envelope of the input signal from the input terminal 10 is compared, at all times, with the envelope of the output signal from the saturation-type amplifier 3, and the error is fed back to the saturation-type amplifier 3, so that the input signal is amplified without giving distortion to the envelope.
With the above described linear amplifier, the detection characteristic of the output signal detector 4 and the input signal detector 5 varies with temperature, so distortion may occur when the temperature varies.
That is, the detection characteristic of the output signal detector 4 and the input signal detector 5 must be linear as shown by a solid line in FIG. 2. But the detection characteristic varies with temperature variation considerably, as shown by a broken line (for a higher temperature) and a chain line (for a lower temperature). As is also seen from FIG. 2, the variation with temperature is larger at the low power region. When the linearity is degraded, the amplitude information in the output signal is distorted. Moreover, this expands the transmission spectrum.